1. Field of the Invention
The present invention relates to a method of milling a thread and including providing a device having a rotatable workpiece spindle for supporting a workpiece on which a thread is to be milled, and a rotatable milling cutter spindle for supporting a thread milling cutter having a work region for milling the thread on the workpiece and provided with teeth having a tooth profile corresponding to a thread profile of the to-be-milled thread, with the workpiece spindle and the milling cutter spindle having their rotational axes extending parallel to each other, and displacing axially the workpiece spindle and the milling cutter spindle relative to each other, while rotating the workpiece spindle and the milling cutter spindle with respective rotary speeds a ratio of which is not equal to 1.
The present invention also relates to a thread milling cutter having a work region with rows of teeth having a tooth profile corresponding to the thread profile of the to-be-milled thread.
2. Description of the Prior Art
The above-described method and apparatus for effecting the method are disclosed in Swiss Document No. 666,429. According to the disclosed method, the workpiece spindle and the milling cutter spindle move relative to each other during a thread milling operation only axially, with the thread million cutter gradually penetrating axially into the workpiece. In order to form a thread having a predetermined length, the work region of the cutter has a shorter length than the length of the to-be-milled thread, and the axial displacement is at least as long as the thread length and, as a rule, exceeds the thread length. The thread is formed during the axial displacement of the cutter, and the time of the axial displacement is relatively large with respect to the displacement distance.
Accordingly, an object of the present invention is to provide a method of thread milling as described above with a shorter duration of the axial displacement of the workpiece and milling cutter spindles relative to each other.
This and other objects of the present invention, which will become apparent hereinafter, are achieved, according to the present invention by displacing the workpiece and the milling cutter spindles radially relative to each other, and by making the distance of the axial displacement shorter than the thread length.
With the inventive method, the thread milling takes place essentially as a result of radial displacement of the workpiece spindle and the milling cutter spindle. The complete length of the to-be-milled thread is obtained as a result of a radial feed of the thread milling cutter which generally accelerates the entire process of the thread milling. The thread milling effected by the inventive method permits to form the thread in two-three times more rapidly than with the method disclosed in the above-mentioned Swiss Patent. The work region of the thread milling cutter used in the inventive method has a length which is at least equal to the length of the to-be-milled thread. The axial displacement according to the inventive method is effected with a ratio of rational speeds of the workpiece and milling cutter spindles deviating from 1 in order to obtain an exactly round and smooth thread. The radial displacement depends on the rotational speed of the workpiece. The present invention consists in the use of a radial feed of the thread milling cutter for effecting thread milling of the rotatable workpiece. The produced chips have an advantageous shape, and the process is characterized by a favorable loading of the thread milling cutter.
The inventive method does not require an angular positioning of the thread milling cutter relative to the workpiece. This permits to use thread milling cutters having a longer work region and permits to mill longer threads. The threads can be milled up to the depth limit. Because of the rotation of the workpiece, the orbital movement of the thread milling cutter is eliminated. The teeth of the thread milling cutter are arranged about the milling axis with a gradient. The inventive method can be used for milling both outer and inner threads. The ratio of rotational speeds of the workpiece and milling cutter spindles varies in a range from 0.8:1 to 1.2:1 and, preferably, in the range from 0.95:1 to 1.05:1. In the later case, a particularly smooth surface of the thread is obtained. However the ratio cannot be 1:1. It is unimportant which of the workpiece spindle and the milling cutter spindle rotates with the larger or smaller speed. Therefore, the speed ratio remains constant during the thread milling process, and the ratio 1:1 never occurs. The rotational speeds are selected dependent on the workpiece. The cutting speed is the same as with the known method. In the point of engagement of the thread milling cutter with the workpiece, they rotate in opposite directions.
It is particularly preferable and advantageous when the radial displacement of the workpiece and milling cutter spindle is effected with the ratio of their rotational speeds being equal 1, and then the axial displacement takes place. During the radial feed, the teeth of the thread milling cutter are loaded uniformly.
It is particularly preferable and advantageous when both the radian and axial displacement takes place simultaneously. This reduces the milling process as the readjusting of the milling device is eliminated.
It is particularly preferable and advantageous when the axial displacement of the workpiece and milling cutter spindles are at least equal to the pitch of the thread divided by the number of teeth arranged about the circumference of the work region of the thread milling cutter. As a rule, the axial displacement is equal to this value. In this case, the axial displacement is minimal.
The present invention is characterized in that the tooth profile of the teeth of the work region of the thread milling cutter differs from the thread profile of the to-be-milled thread, and the tooth profile is corrected with respect to the thread, and the teeth are arranged with a respective pitch. The type and the amount of the deviation of the tooth profile from the milled thread profile, because of the parallelism of the workpiece and milling cutter axes, is determined by the pitch of the to-be-milled thread.
With respect to the correction of compensation of the tooth profile, the following can be said. The thread milling cutter or the thread-forming teeth of the cutter rotate in a plane that is not identical with the plane which in each arbitrary infinitesimal small point extends parallel to the produced workpiece profile. Therefore, the outline of cutter profile geometry in the normal plane (perpendicular to the pitch) does not coincide with the workpiece geometry in this plane. Rather, the cutter profile geometry, because of difference of the directions of the operational planes, would be projected onto the workpiece. In order to obtain a desired geometry, the cutter geometry should deviate from the projection of the cutter profile geometry by a projection identical in magnitude but having an opposite direction. Thus, the cutter and the workpiece have different geometries in the normal plane. Thus, the cutter profile geometry is determined by the to-be-produced workpiece profile geometry and by the projection of the position of the operational planes. This process is called profile compensation.
The thread milling cutter according to the present invention is determined by its own shape and by additional data which are given on the cutter itself and/or are printed on the cutter package, accompanying document, or prospectus. The additional data includes the thread profile of the to-be-milled thread with respect to which the tooth profile is corrected, and an indication that the cutter can be used in a thread milling device having only four axes (and no additional inclination axes). Thus, the thread milling cutter according to the invention is adapted for use in a thread milling device having two parallel rotational axes and two feeding directions.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.